The principle of an miniature optical frequency standard based on traditional calcium atomic beam is that, when a calcium oven is heated up to a temperature around 700 centigrade, the heated calcium atomic beam is ejected from the calcium oven and passes a 657 nm laser traveling wave field generated by two pairs of cat eye devices which are parallel with equal interval orderly after collimation. A proper magnetic field is added to the Ramsey interaction region so as to separate different magnetic sublevels in 3P1 state. The 1S0 ground state atoms coming out from the calcium oven is stimulated by π pulse of polarized 657 nm laser in the Ramsey interaction region. The atoms stimulated to mF=0 sub-level in 3P1 state may give off 657 nm fluorescence by spontaneous emission transition. A photoelectric detector measures the 657 nm fluorescence given off by the spontaneous emission transition of the atoms in an excited state at the downstream of atomic beam flow. The miniature atomic clock at optical frequency based on the traditional calcium atomic beam is shown in FIG. 1.
So far, all the atomic clocks of optical frequency based on a heated atomic beam technology are limited by the low signal-to-noise ratio of fluorescence signals with no exception, so that the stability can not be improved and the accuracy that may be achieved is also limited ultimately. At present, the best accuracy that the atomic clock at optical frequency based on the calcium atomic beam may achieve is a little worse than that of a HP5071 small cesium clock so that it can not compete with the commercial HP5071 small cesium clock. The main reason is that the probability of the spontaneous emission of the atoms, only about one thousand photons per second, is very low after the region of 657 nm clock transition interaction used for detection, plus the limited fluorescence collecting area for detection makes that the detection efficiency for the atoms is so low to only around 1% or less. Such low detection efficiency for the atoms greatly limits the accuracy and the stability of the atomic clock that can be implemented.